Macrophages and microglia are the central cells in the development of HIV dementia (HIVD): macrophages may be responsible for transporting virus into the central nervous system (CNS), and they and microglia are the major cell type harboring HIV sequences. Macrophages and microglia may be responsible for the production of neurotoxins and potentially toxic viral gene products like gp12O and tat, and if neuroglia and neurons are infected to any great extent, microglia are undoubtedly the source of virus. One aspect of neurotropism, the ability to preferentially replicate in microglia, has been identified by studying a large panel of isolates obtained at the time of primary HIV infection. With these, as well as viruses isolated from the brains of individuals with HIVD, we propose to study the microglial-virus interactions important in the development of HIVD in three specific aims. In the first aim we will (i) determine the mechanism of accelerated microglial replication using PCR to delineate the sequential steps of the infectious process and (ii) define whether microglial-tropism is determined at the level of cell entry (iii) determine why microglia are inherently more susceptible than MDM to HIV infection and (iv) explore the effects of the state of activation of microglia, and the influence of cytokines present in brain, like tumor necrosis factor alpha (TNFalpha) and interleukin 1 beta (IL1beta) on viral replication. These experiments will test the hypothesis that microglial tropism is a phenomenon whose mechanism can be clearly defined by analyzing the series of steps involved in viral replication. In the second aim we will define the genetic underpinnings of preferential replication in microglia, and in the third aim we will determine whether neurovirulence is strictly correlated with increased replication by studying the effects of viral infection with primary isolates from blood and brain on microglial secretion of potential neurotoxins like TNFalpha and platelet activating factor (PAF). However, since the critical neurotoxins may not yet have been identified by previous strategies, we will also use differential display, a technique that may detect mRNAs that are either increased or decreased in infected microglia, and has the advantage that it will provide an unbiased look at the system. The results of these studies should help delineate the rational treatment of HIVD.